Re: Read Performance issue when Xen Hypervisor is activated

[Dario Faggioli <dario.faggioli@citrix.com>]

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[Cc-ing someone which have done disk benchmark in somewhat recent time]

On Tue, 2016-12-27 at 14:26 +0000, Michael Schinzel wrote:
> We have searched in the last days more and more for the cause of this
> performance issue.
>  
> In cooperation with the datacenter, we change some hardware to check,
> if the problem already proceeds. We put the RAID Controller included
> all RAID Arrays to another Supermicro Mainboard: X10SLM-F with only
> one CPU. The result was, we got 400 MB/s read Speed. So it seems
> there is an issue with the Servers Mainboard / CPU and the Xen
> Hypervisor but, we also change the Mainboard to an Supermicro X9DR3-F 
> with the actual BIOS Version 3.2a – these also do not solved the
> problem with the performance.
>  
> What we also have done:
> -          Upgraded Hypervisor from default Debian 8 – 4.4.1 to 4.8.
> -          Tested some kernel boot configurations\
>
I think it would be useful to know more about your configuration, e.g.,
are these tests being done in Dom0? How many vCPUs and memory does Dom0
have?

> With an non hypervisor Kernel, the system also uses the read Cache of
> the controller and after some read operations at the same file, it
> gets 1.2 G/s back from the Cache. At Xen Hypervisor Kernel, it seems
> the system do not use any caching operations. I also tested a bit
> with hdparm:
>  
> root@v7:~# hdparm -Tt /dev/sdb
>  
> /dev/sdb:
> Timing cached reads:   14060 MB in  1.99 seconds = 7076.16 MB/sec
> Timing buffered disk reads: 304 MB in  3.01 seconds = 100.85 MB/sec
>  
> This Performance is horrable. It is a RAID 10 with read/write cache
> and SSD Caching functions.
>  
> Does somebody know how Xen proceeds with such Caching Systems?
>  
>  
> Yours sincerely
>  
> Michael Schinzel
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
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Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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